[Key Vault] Regenerate with latest autorest

[mccoyp]

Description

Regenerates code for all packages on the same swagger commit as before (https://github.com/Azure/azure-rest-api-specs/commit/e2ef44b87405b412403ccb005bfb3975411adf60), using the latest version of autorest. The updated generator no longer uses six or msrest, which means that we can drop the former requirement after https://github.com/Azure/azure-sdk-for-python/pull/26150 is merged, and can drop the latter requirement as part of this PR 🙂

All SDK Contribution checklist:

• [x] The pull request does not introduce [breaking changes]
• [x] CHANGELOG is updated for new features, bug fixes or other significant changes.
• [x] I have read the contribution guidelines.

General Guidelines and Best Practices

• [x] Title of the pull request is clear and informative.
• [x] There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• [x] Pull request includes test coverage for the included changes.